The process for producing alkali metal silicates by alkaline fusion of silica is well known, in particular from "Soluble Silicates" by J. G. Vail, Reinhold Pub. Corp., Vol. 1, p. 6 (1952). This process is still virtually the only process used today for producing alkali metal silicates.
Another well known process is the autoclaving of silica with an alkaline solution. Thus "Gmelins Handbuch der anorganischen Chemie," Vol. 21 (1928), p. 861, discloses work of Liebig (1857) and other authors along these lines, but these processes only produce silicates that are too rich in sodium hydroxide for industrial use.
Patents such as British Pat. No. 788,933 show the advantage of a process working at a temperature between 175.degree. C. and 320.degree. C., whereas alkaline fusion requires temperatures near or exceeding 1,300.degree. C.
U.S. Pat. No. 3,971,727 describes the production of alkali metal silicates in aqueous solution under pressure at a temperature of between 138.degree. C. and 210.degree. C., but is necessary to agitate mechanically and to filter the product. The process involves fairly long reaction times, the presence of by-products in the reaction product, and is thus not a commercially feasible process.
European Pat. Nos. 33,108 and 33,109 also disclose processes for the production of sodium silicates, but these processes also require agitation and filtration of the suspension.
A process for reacting silica with an alkaline solution to produce alkali metal silicates which can save energy is of considerable value. In order to produce alkali metal silicates economically and save energy, the process must produce solutions of silicates sufficiently rich in SiO.sub.2 which can be directly used commercially. For example, in the production of sodium aluminosilicates (type A zeolites) and for the detergent industry, sodium silicate having an SiO.sub.2 /Na.sub.2 O weight ratio near or greater than 2 and, at most, equal to 2.5, is required.